bug#23679: 25.0.94; vc-region-history does not work correctly when narrowed

[Ivan Andrus]

1. Emacs -Q
2. Open a file with git history, say something in the Emacs repository.
3. Narrow to a region not at the beginning of the file
4. Select a region
5. M-x vc-region-history
6. Notice that the history shown is for the beginning of the file

This is because vc-region-history uses line-number-at-pos which returns
the line number in the narrowed portion of the buffer.

We could either put a save-restriction/widen pair in vc-region-history
or create a function (say widened-line-number-at-pos or
file-line-number-at-pos), which would be generally useful IMO.
However, that would lead to two calles to save-restriction and widen
instead of just one.  Given the recent discussion on emacs-devel about
widening for programs I felt I should get some feed back on the
preferred method before pushing a fix.

-Ivan